Pulverizers such as bowl mills are commonly used to prepare coal for introduction into the combustion chambers of steam generators; representative pulverizers are currently offered for sale by Babcock and Wilcox, Foster-Wheeler and Combustion Engineering. Bowl mill pulverizers typically perform a classification function through the use of a vertical air flow through a "throat" which is made up of a circular arrangement of pitched vanes surrounding the outer periphery of the pulverizing surface and forming air flow passages between a wind box and the classification area. The vanes are made up of metal plates usually welded to and between inner and outer rings. The vane assembly or "throat" may be stationary or it may be mounted for rotation about a vertical axis.
Air flow rate through the passages formed by the pitched vanes is a function of the effective cross-sectional area of the passages and the pressure head produced by the fans, turbines or other air drive mechanisms. It is desirable to control air flow rate through cross-sectional area adjustment to optimize pulverizer performance.
One prior art mechanism for controlling cross-sectional area and flow rate comprises spacer blocks which are bolted to the inside ring of the vane assembly. The blocks can come in various sizes or may be bolted on top of one another to reduce the size of the air flow passage and the air flow velocity. In this approach the spacer blocks are in the path of particulate matter flow and, therefore, are subject to abrasion and wear. As a consequence, the spacer blocks must be made of a more expensive wear resistant material. Moreover, it is a time consuming and cumbersome job to install and remove the spacer blocks.
An alternative approach to air flow control is disclosed in my U.S. Pat. No. 4,907,751, "Rotating Throat for Coal Pulverizer", issued Mar. 13, 1990. In that patent I disclose the use of slide-on, wear resistant vane liners in the form of metal plates which overlie the upper principal surface of the pitched vanes. Each liner plate has an integral angled portion which rests on the top edge of the vane and partially closes the air flow opening. The vane liners are held in place by means of arcuate overplates or caps which are bolted to the top surface of the inner portion of the vane/throat assembly. The degree to which the arcuate plates extend over the openings also affects the area of the air flow passage and the air flow rate. Like the spacer blocks, adjustment or change in air passage size can be achieved only by interchanging one set of liners or caps for others of a different size.